1. Technical Field
The subject matter described herein generally relates to systems and methods for performing echo cancellation in an audio communication system, such as a telephony system.
2. Description of Related Art
Acoustic echo is a phenomenon that occurs in audio communication systems, such as telephony systems, when speech generated by a remote or “far-end” speaker and played back over a local or “near-end” loudspeaker is picked up by a near-end microphone and transmitted back to the far end. The transmitted signal is a delayed version of the original, which causes the echo. The received far-end signal does not transfer directly from the loudspeaker to the microphone, but is subject to the environment in which the loudspeaker and microphone are located. This may include differing signal paths causing reverberation and spectral shaping. These effects are the transfer function between the loudspeaker and the microphone. Such transfer function is dynamic, as objects in the environment move or the loudspeaker and/or microphone change position.
Acoustic echo cancellation refers to a process by which the acoustic echo is modeled and then subtracted from the signal that is to be transmitted to the far end. Traditionally, acoustic echo cancellation is performed using an adaptive filter to estimate the transfer function between the loudspeaker and microphone. For such acoustic echo cancellers there is always a risk of divergence and the possibility of objectionable artifacts. Furthermore, for such acoustic cancellers, it is necessary to detect periods when the near-end and far-end speakers are talking simultaneously (a condition known as “double-talk”) and to stop updating the adaptive filter during such periods to prevent divergence.
Many approaches to acoustic echo cancellation rely on the assumption that the echo path can be modeled by a linear filter. However, it is possible that certain components, such as the near-end loudspeaker and/or an amplifier associated therewith, will introduce non-linear distortions and thus the echo path must be modeled as a non-linear system. A non-linear system transfers energy from one frequency to other frequencies, and so the response will contain combinations of the input harmonics. To address this, acoustic echo cancellers have been developed that attempt to model non-linear distortions and remove them from the signal to be transmitted to the far end. However, such non-linear acoustic echo cancellers are typically exceedingly complex and therefore impractical to implement.
The features and advantages of the subject matter of the present application will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.